Method of purifying sulfuric acid



J. F. SKELLY EVAL METHOD OF' PURIFYING SULFURIC ACID Filed July 8, 19554 ATTORNEYS Dec. 9, 1958 United States Patent METHOD oF PURIFYINGSULFURKIC ACID Application July 8, 1955, Serial No. 520,822 s Claims.(ci. 2317z The present invention relates to novel methods of puri,-fying sulfuric acid, and-more particularly, pertains to purifyingsulfuric acid by fractional crystallization. This application isacontinuation-in-part of application Serial No. 310,974, filed September23, 1952, and now Patent No. 2,716,592, issued August 30, 1955.

At present, sulfuric acid is used extensively for various purposes, suchas refining lubricating oils, alkylation of hydrocarbons, dehydratingwet materials, etc. Usually, the acid becomes contaminated withmaterials of the particular process in which it is used, and in order torestore its purity and/or strength, the acid is subjected to apurification treatment. For example, with respect to the case ofalkylating hydrocarbons with sulfuric acid as the catalyst, the acidstrength should be maintained at a high level and the acid should bereasonably free of certain impurities, otherwise the quality of thealkylate product is seriously affected. Prior to our invention, it wasthe practice to purify sulfuric acidby one of two methods. The lirstmethod involves dilutingk the spent acid with water, until an oil orwater insoluble layer forms, and then separating the two phases bydecantation, etc. The obvious disadvantage in the process is thatextensive refortilication with S03 or fuming sulfuric acid is requiredin order to restore acid strength. The other method is the rathercomplicated carbonization technique which involves burningthe spent acidto remove carbonaceous material and thus produce SO2 and S03 gas,convert the SO2 to S03, which, after separation and purification, isblended with water to make sulfuric acid again. For various reasons, theabove-described methods are not entirely satisfactory, hence manyworkers are diligently seeking more effective and economical methods ofsolving the problem of purifying spent sulfuric acid.

Therefore, it is an object of the present invention to provide a novelmethod of purifying sulfuric acid.

Another object of :the present invention is to provide a novel methodfor the removal of substantial amounts of non-polar and/ or polarimpurities from sulfuric acid by fractional crystallization. Y

Still another object of our invention is to provide for the purificationof sulfuric acid by fractional crystalliza- Ition throughauto-refrigeration means.

Other objects and advantages of our invention will become apparent fromthe following description and explanation thereof.

The present invention is concerned with a method of purifyingcontaminated sulfuricracid containing at least one impurity selectedfrom polar `and non-polar impurities which comprises passing alow`viscosity organic liquid which is chemically inert' and immisciblewith sulfuric acid to a crystallization zone and thus forming a liquidbody therein, passing the contaminated acid to the crystallization zone,dispersing contaminated acid into the liquid body in the crystallizationzone which is maintained-.at a temperature sufcient to crystallizesulfuric acid, withdrawing a mass of acid crystals and liquid from thecrystallization zone, recycling a portion. of said mass to thecrystallization zone, separating acid crystals from the remaining massand melting the crystals to produce an acid of improved purity.

In another embodiment of the invention, a volatilizable organic liquidis employed in the crystallization zone and the temperature ismaintained by vaporizing a portion of the organic liquid, the vaporizedorganic liquid is recompressed, and then it is cooled by indirect eX-change with the acid crystals, thus the recompressed organic liquid isrecondensed and the acid crystals are melted.

In still another embodiment of the invention, a portion of the organicliquid which is separated from the acid crystals is subjected to heatingin order to vaporize the organic liquid and leave a residue of impurityoriginally contained in the acid.

Another embodiment of the invention is concerned with the operationinvolving the use of a volatilizable organic liquid spacer and whereinvaporized organic liquid is collected from (a) the crystallization zone,(b) the separating zone in which acid crystals are separated vfrom theremaining liquid, (c) the melting zone in which acid crystals aremelted, and (d) Vthe separating zone in which mother liquor is separatedfrom the spacer material in a liquid condition, and then the collectedvaporized spacer is combined prior to recompression in order to maintainthe desired composition of spacer material when two or more differentvolatilizable compounds are employed therefor.

In the practice of purifying sulfuric acid by crystallization, it isfound that unless the acid feed is seeded, crystallization may not occurat the expected temperature, but at some lower temperature. Thesupercooling Vof acid results in a higher operating oost for theprocess, because more cooling is required to maintain the temperature ofcrystallization. To avoid this disadvantage, it is proposed by means ofthis invention to recycle part of the mass of acid crystals and liquid,to a point 4which is preferably near :the point of introduction of acidfeed to the crystallization zone.

Another important advantage in our invention resides in the method bywhich impurities present in the spacer material, followingcrystallization, are removed to avoid a continuous build-up of the samein .the system. This method involves heating a portion of the separatedliquid spacer to drive olf the spacer and thus leave a residue ofimpurities. In this connection, the spacer has a lower boiling pointthan any of the impurities, otherwise the technique would not beelfective. Generally, the spacer boils at about 50 to about 500 F., moreusually about to 200 F., lower than the impurities. The vaporized spacercan be then re-condensed and used in the system.

In the system involving the use of a volatilizable spacer material, thespacer material which becomes vaporized is recompressed and then cooledby contacting the same indirectly with acid crystals for exchange ofheat. Thus the compressed spacer material is condensed and the acidcrytals are melted. In the present invention, the cost of coolingrepresents a major item of expense, hence, it is important to conservethe low temperature state as much as possible. The exchange of heatbetween compressed spacer material and acid crystals has manyadvantages, among which is the absence of contamination of purified acidby reason that heat exchange is indirect. Further, in the presentinvention, where a mixture of compounds is employed as thevolatilizablespacer, it is important to combine all the vaporizcd spacer in order tomaintain the desired composition. The combining o f vaporized spacer iseffected prior to compression in order to avoid complicated and/ orcostly equipment for the purpose.

The present invention is concerned with purifying sulfuric acidsolutions which contain polar and non-polar impurities. For the purposeof this specification and the appended claims, water will be treated asa separate impurity in addition to other impurities. The contaminatedsulfuric acid solutions may contain any amount of sulfurie acid, waterand other impurities and still be satisfactory for processing under thepresent invention; however, our invention has particular utility as amethod for purifying strong acid solutions, namely, solutions containingat least about 80% total acidity expressed as H2SO4, preferably morethan about 90 on the same basis. The individual impurity discussed abovemay comprise the sole contaminant in the acid solution, or the acid maycontain at least two impurities `and Vthe concentration of each impuritymay vary widely in relative proportion to each other. In general, thecontaminated acid to be purified contains about to 20%, or 0.01 to 10%,by weight of polar impurity, about O to 25%, or 0.01 to 10%, by weightof non-polar impurity and 0 to 10%, or about 0.1 to 5%, by weight ofwater.

The contaminated sulfuric acid solution may be subjected directly tocrystallization temperatures. In such instance, the quality of crystalsformed will depend upon the amount and type of impurities which arepresent in the original acid solution. The nature of the impurities willaffect the concentration of acid in the acid crystals as well as thetemperatureat which such crystals will form. The type of impurity willalso aiect the quantity of sulfuric acid remaining in the liquid phaseand the temperature at which the crystals form. The amount of impuritiesalso influences the purity of the crystals, and the freezing pointtemperatures at which crystals are formed. Ordinarily, for any sulfuricacid solution, for given concentrations of impurities, it is found thatcrystals of sulfuric acid of a xed purity will continue to form as thetemperature is decreased, until aeutectic point is reached, whereatadditional crystals of different composition form with some of theimpurities still remaining in the liquid phase (this liquid may besoluble or miscible in the carrying or supporting media or may betotally immiscible). As the temperature of crystallization iscontinuously decreased and crystals of a constant purity are formed,there is a continuous decrease in the concentration of acid in theuncrystallized liquid, commonly known as mother liquor. In some cases,depending upon the nature of the impurities, there may be more than oneeutectic point, consequently, it is desirable to practice our inventionwithin the limits imposed by the eutectic characteristics of the acidsolution or pre-treat the original contaminated acid solution so thatcrystallization is confined to sulfuric acid (water-free) leaving theimpurities in the liquid phase within the temperature range at which thedesired acid purity is obtained. The lowest acid concentrationobtainable in the mother liquor will depend upon the eutectic point ofthe spent acid solution.

Of the impurities usually found in sulfuric acid, water appears to havethe greatest effect on the concentration of sulfuric acid in the motherliquor. This characteristic is to be expected, since sulfuric acid has avery strong affinity for water. In view of this factor, our inventioncan be practiced by regulating the water content of the contaminated orspent acid in order to control the lossof sulfuric acid in the motherliquor. However, it should not be understood from the above statementthat impurities other than water do not exert an influence on thefreezing point characteristics of the acid solution, because it isintended to show a practical method of obtaining high purity acidcrystals. Therefore, it will be noted that the present invention is notlimited to any range of water concentration in order to produce highpurity acid crystals, because the amount of water in the contaminatedacid can be controlled by treatment with an S03 containingdmaterial, e.g., sulfur trioxide and/ or fuming sulfuric aci At crystallizationtemperatures, it is discovered that the polar and/or non-polarimpurities remain predominantly in the mother liquor and, to a lesserextent, they are present in the spacer material. Usually, part of theimpurities are sorbed on the surfaces of the said crystals and to someextent occluded by the crystals. It was observed, in some instances,that there is a correlation between crystallization temperature andpurity of said crystals. As the temperature `is lowered, the crystalsbecome less pure, and this phenomenon is apparently caused by theimpurities undergoing an increase in viscosity with lowering oftemperature and, hence, readily sorbing on the surfaces of the acidcrystals. The effect is counteracted by crystallizing the contaminatedacid solution in the presence of the organic liquid which does notsolidify at crystallization temperatures and is preferably substantiallyless viscous than the impurities at corresponding temperatures. Ingeneral', the vspacer material has a viscosity of about 0.1 to 20centipoises, preferably about 0.5 to 2.5 centipoises. Furthermore, sucha spacer liquid is chemically inert and substantially immiscible withsulfuric acid and preferably possesses the property of unfavorablyinfluencing the sorption of impurities on the crystal surfaces. For ourpurposes, the use of a relatively low viscosity, inert, immiscible,organic liquid serves to space the impurities from the acid crystals orsuch a liquid tends to adversely inuence the aflinity of the impuritiesfor the crystals. Itis preferred that the low viscosity spacer liquidshould have little or no 'attraction for the sulfuric acid, such as anundesirably high solubility, otherwise a separation of the `acid fromthe'low viscosity liquid will need to be-effected. The use4 ofthisliquid as a spacer during Vthe crystallization step makes possible theemployment of lower temperatures of crystallization, which heretoforewere not Apossible or practicable, because of the diiculty inseparatngcrystals from liquor. Generally, the organic liquid used as aspacer in relation to the sulfuric acid in liquid form, on a volumetricratio basis, is about 0.5 to about 1000 parts of spacer per part ofacid. The preferred procedure is to disperse the liquid acid in acontinuous phase of spacer liquid, in order to provide a condition inwhich Aa large amount of spacer is present relative to the liquid acidduring the crystallization step. Accordingly, for such a procedure, itis preferred to employ about 2 to about 25 parts by volume of spacerliquid per part of acid which is present in the crystallization zone.

The crystallization of .contaminated acid is conducted at atemperaturein the range of about 50 F. to about -l00 F. The invention isparticularly applicable for purifying .sulfuric acid at low temperaturesbelow 0 F., for example, -infthe range of about 40 to 0 F. The selectionof a crystallization temperature depends upon the type and quantity ofYimpurities present. The temperature selected will generally be thatresulting in maximum crystal yield with minimum impurities. Lowertemperatures generally favor larger yields of acid crystals, however,simultaneously there may be the adverse effect of increased motherliquor viscosity to lower the crystal purity at these lowertemperatures. The presence of our spacer liquid tends to overcome thiseffect. The optimum temperature range to be used will depend upon thenature of the impurities and the desired extent of purification.

The separation of acid crystals from the mother liquor ordinarilypresentsv diicult problems when low temperatures are employed for thecrystallization. It was found that in the case of mechanical separationof crystals from mother liquor, the better procedure is to employcentrifugal separation. impurities present in the spent acid tend toadhere to the crystal surfaces, and centrifugal separation appears toelfect better removal of these irnpurities vfroml the -s'urfaces of thecrystals than some of the other mechanical methods. The separation ofcrystals from mother liquor can be made much easier by dispersing tinedroplets of acid to be purified int-o the liquid which serves as thesupporting or spacing medium. The temperature of the liquor may exist atthe desired crystallization temperature, consequently, the droplets ofacid are crystallized as small particles. The separation of crystalsfrom the spacer liquid is preferably conducted by centrifugalseparation, and during the separating operation, the spacer liquidserves to wash the impurities from the crystal surfaces. The separationof crystals from the supporting medium can be effected by other meanssuch as, for example, decantation, filtration, etc., and the crystalscan be washed with a fresh spacer liquid which can be more effective forthe removal of impurities from the crystals.

After the crystals are separated from the uncrystallized liquid, theycan be washed with a liquid solvent which is chemically inert withresp-ect to sulfuric acid and preferably miscible with the impuritiessorbed on the crystal surfaces and immiscible with the sulfuric acid;or, if such solvent is not available, a low Viscosity liquid which ischemically inert and immiscible with the acid can be used. In theWashing step, it is preferred to have turbulent conditions or agitationin order to promote the removal or desorption of impurities from theacid crystals. The washing operation can be repeated as frequently as itis desired, the only limitation being an economical consideration.Generally, about l to about 1000 parts by volume of wash liquid,preferably about 5 to about 25 parts by volume, per volume of solid acid(crystals) are employed in the washing operation.

Cooling of the acid can be effected by means of autorefrigeration whicheliminates exchangers and scraper coolers and allows proper crystaldispersion and size. Ordinarily, the technique of auto-refrigeration isaccomplished by employing a liquid in the crystallization zone which canbe vaporized by absorbing heat at relatively low temperature levels.Such a liquid facilitates temperature control by regulating the pressurein the crystallization zone. In practice, the pressure of the system canbe superatmospheric, atmospheric or subatmospheric, depending upon thevapor pressure characteristics of the liquid. Usually, this .pressure isabout 5 to about 50 p. s. i. a. and the selection thereof dependsprimarily on the type of volatilizable liquid and the coolingrequirements. Generally, a volatilizable liquid, preferably chemicallyinert and immiscible with H2SO4, is suitable. The liquids can beselected from a variety of classes including light aliphatichydrocarbons having about l to`7 carbon atoms, halogenated aliphatichydrocarbons, halocarbons, aliphatic ethers, etc. Specific examples ofvolatilizable liquids are the light parains, e. g., methane, ethane,propane, n-butane, isobutane, isopentane, npentane, etc., or mixtures ofthe foregoing. It will be noted that the volatilizable liquid serves asthe low viscosity liquid which is discussed hereinabove as the spacer orsupporting medium and also as the wash liquid. For example, propane andisobutane mixture is found to be an excellent and relatively cheapliquid for providing auto-refrigeration, serving as a spacer duringcrystallization of the acid and the wash liquid for turbulentlycontacting or washing the crystals.

To better understand the nature of our invention, speciiic illustrationsof the various concepts will now be given by reference to the drawingwhich forms apart of this specification.

The drawing contains a specific embodiment of a process for purifyingcontaminated sulfuric acid.

. As a result of alkylating an olefin and an iso-paraflin, the sulfuricacid becomes contaminated with mono-alkyl sulfate, dialkyl sulfate, anda relatively high molecular weight polymeric material. In addition, as aresult of oxidation of ester to SO2, water and polymer, the watercontentof; the acid solution increases. The 'ecacy of presence of thenon-polar impurities, e. g., the dialkyl sulfate and polymer and water,and so it is necessary to decrease the concentration of these impuritiesin order to avoid any serious eect on the quality of the alkylateproduct. According to one theory of the isobutane-olefin reactionmechanism, the mono-alkyl sulfate appears to have no adverse effect onthe alkylation reaction, but rather, it appears to be an intermediatematerial in the production of the alkylate. Water serves to dilute theacid solution possibly resulting from entrainment or solution with thehydrocarbon stream entering the alkylation reactor or frompolymerization side reactions of the dialkyl ester sulfate, theseby-products accumulate in the acid and retard its catalytic activity andlower the qualit-y of the alkylate. In commercial practice, when all theimpurities increase to 10-l2%, the catalytic activity of the HZSO., isconsidered too low, and is Withdrawn from the reaction zone. If allowedto get below this value, the olefin polymerization reaction becomesexcessive and the acid deteriorates rapidly. The spent acid isregenerated as illustrated below.

In the drawing, spent acid from an alkylation process is receivedthrough line 10 at the rate of 350 barrels per day, b. p. d. The spentacid has a titratable acidity of 95.1% by weight of sulfuric acid. Itshould be understood that the titratable acidity includes the freesulfuric acid as well as the ionizable hydrogen of the mono-ester. Inthis example, the spent acid has the following composition:

Component: Weight percent H2SO4 89 Mono-ester (CJ-I9) HSO4 8.1 Water 2.9

The spent acid is fed through line 10 and the stream is divided so thatpart passes through line 12 and the rest passes through line 14 at therate of 200 b. p. d. The spent acid flowing through line 12 iscontrolled by means of a ycontrol valve 16. This spent acid is cornbinedwith regenerated or purified sulfuric acid by means of an acid productline 18. In this example, the regenerated acid has a titratable acidityof about 99% by weight of sulfuric acid. This regenerated acid lis toohighly concentrated for the temperature at which the alkylation reactionis conducted, because acid -crystals would form. Consequently, to avoidsuch an occurrence, part of the spent acid is recycled with the purifiedacid in a quantity which will provide a nal concentration of about to96% by weight of H2804.

The spent acid owing through line 14 is then mixed with fresh acidcontaining sulfur trioxide via a supply line 2) to chemically combinewith substantially all of the free water. The presence of free water inthe crystallization zone causes an undesirable quantity of sulfuric acidto remain uncrystallized in the mother liquor. This phenomenon occurssubstantially less with respect to the non-polar impurities. Generally,combining the water with sulfur trioxide or highly concentrated sulfuricacid improves the yield of acid crystals substantially. When the heatliberated by the absorption of S03 becomes unduly high, the schematicflow can be modified to provide the passage of the combined streams offresh acid containing SO3 and spent acid through a cooling means, e. g.,a water cooler or exchanger (not shown).

The fortified spent acid in line 14 then passes to the low pressurenozzle of an eductor 22. A'stream of propane and isobutane liquid isalso fed to the high pressure nozzle of eductor 22 via a supply line 24.The

propane-isobutane mixture, hereinafter also called propane, is used as asupporting media for the liquid and solidilied acid, a cooling media tomaintain the crystallization temperature by removing heat throughvaporization, and as a spacing media to flush viscous impurity materialfrom the acid crystals'. -The liquid propane-isobutane mixture is fed tothe eductor where the fortified spent acid is introduced in the liquidphase and dispersed therein as small droplets. The volume ratio ofhydrocarbons to fortified spent acid can be varied from about 2 to`about 25:1 or specifically 4:1. The dispersion of spent acid inpropane-isobutane serves to facilitate the crystallization of sulfuricacid by providing a maximum surface area for the flow of heat from thedroplets of acid, andthe smaller the droplets the better the operation.The heat of fusion is removed by vaporizing part of the supportingmedium. This dispersion or emulsion of spent acid in propane passes fromthe eductor 22 and enters another mixer 25 inside the crystallizer 26via line 28. The emulsified acid mixes with seed crystals from line 32in this4 mixer and the resulting mixture enters the crystallizer 26which is at a temperature in the order of about 25 F. This temperaturemore usually may vary for parainic hydrocarbons in the range of about 40to F., preferably about 35 to 20'F. The spent acid, after fortification,is at a temperature of about 30 to 50 F., or specifically 40 F.,andvaporization of a portion of the liquid propane-isobutane mixture,which is at its boiling point, gives the final temperature mentionedabove.

Inside the crystallizer, the emulsion of acid and propane is fed to thelow pressure nozzle of the second eductor 25. Propane liquid containingacid crystals from line 32 passes to the low pressure nozzle of 25 andmixes with the emulsion from line 28. The crystals of acid serve toinitiate crystallization of the liquid acid and avoid supercoolingeffects. This practice is commonly known as seeding the solution. Therecycle stream of acid crystals is circulated at a rate of about 2 timesthe holdup of liquid in the crystalizer 26 every minute. Thecrystallization temperature is obtained by controlling the pressure inthe crystallizer at about p. s. i. g. At this pressure, part of theliquid propane-isobutane vaporizes and passes overhead from thecrystallizer through lines 30 and 31 and then to compressor 33 by which`the pressure is regulated. The heat of vaporization is absorbed from thecrystallizing acid and liquid present, thus causing the temperature tobe maintained at the desired level.

The maintenance of temperature in the crystallizer Ican be accomplishedby other means, viz., directly or indirectly. By the indirect means,cooling materials such as cooled liquids or gases can indirectlyexchange heat through vertical or horizontal tubes or coils in thecrystallizer. The direct means involve, for example, the passage ofcooled fluids or solids through the crystallizer. Finely divided solidscan serve to cool the contents of the crystallizer and also seed thesolution. The cooled fluids for direct cooling can be a gas or liquidwhich does not freeze at crystallization temperatures.

A slurry of sulfuric acid crystals is withdrawn from the crystallizer bymeans of a bottom line 38. The slurry of acid crystals is conveyed to abasket-type centrifuge 40 by means of pump 42 and lines 44 and 46. Aportion of the slurry of crystals is recycled to the high presure nozzleof eductor 25 in the crystallizer as previously described via line 32.The rate of recycle is Icontrolled by means of control valve 48 in line32.

The basket-type centrifuge is a known type of apparatus. In thiscentrifuge, the acid crystals are retained on a spinning perforateddrum. The uncrystallized liquid comprising sulfuric acid, water, monoanddi-esters, propane and polymer is forced through the cake or drum bycentrifugal force and then withdrawn from the outer casing of themachine. A leveling rake rides on the crystal cake for distributingevenly the solids on the drum. When a-predetermined cake thickness isreached, the rake arm closes the slurry inlet valve. A time cyclecontroller then introduces a wash liquid onto the cake for removal ofany sorbed impurities on the crystals. The wash liquid flows from apredetermined period, followed by further spinning of the drum to insuremore complete separation of' liquids. Thereafter, a knife rises andpares the cake of crystals from the spinning drum and dlverts them to asuitable outlet. A thin layer of crystals does remain in the drum, andthis can be further washed, before opening the charging valve to repeatthe cycle.

Referring to the drawing, the uncrystallized l1qu1d 1s discharged fromthe centrifuge through a bottom lme l52. This uncrystallized liquid ispassed into the bottom sectlon of a separator 54. The liquidpropane-isobutane contarning some heavier hydrocarbon or polymerseparates as a top or upper liquid phase in the separator, becausepropane and isobutane are not readily miscible with sulfuric acid-watermixtures and to only a small extent with the esters. The lower liquidlayer comprises this heavier immiscible liquid of sulfuric acid, waterand monoand diesters. The materials in the lower layer are withdrawnfrom the separator through a bottom line 56 at the rate of 20 b. p. d.In this particular example, it is not feasible to attempt recoveringtheacid from the bottom product inline 56. However, in those cases wherethe fortification with S03, etc., does not completely react with all thewater, or no fortification step is used, it is contemplated, within thescope of this invention, to fortify the bottom product in line 56 of theseparator to recover additional purified acid throughre-crystallization. Any propane vapors in the separator resulting fromheating, mechanical friction, etc., are removed through an overhead line58, which connects with the propane vapor line 59.

The liquid propane in the separator 54 is withdrawn by means of line 60,and part of this propane stream is recycled to-the centrifuge forwashing the crystal cake by means of lines 62 and` 64. The propane isconveyed by. means ofV pump 66. The washing operation should beconductedat a temperature suiiciently low to avoid melting the crystals.This will be determined by the qualityy of the crystals andltherespective crystallization temperature. In this example, `the washliquid is at 35 F., and the washing isl conducted by the continuouspassage of propane through the cake of crystals of the same compositionand temperature as that in the crystallizer. When using a batchwiseprocess, the crystals can be washed as many as 10 times or more withequal parts of low viscosity liquid. A slip stream of propane which isnot used for washing is passed through a line 68 containing a regulatingvalve 69 to evaporator 70 (or other fractionation equipment) where anyimpurities such as esters and polymer are separated `from the propane.Propane vapors leave the evaporator 70 via `a vapor line 72 and passthrough lines 59 and 31 before entering the propane compressor 33. Theliquid esters and polymer leave the evaporator byline 73. Thisseparation of esters and polymer may also be accomplished by returningthe stream `from line 68 to the alkylation plant.

The washed crystals are discharged from the centrifuge by scraper 76,which in turn is connected to a line 78. The line 78 is connected to amelting vessel 80 in which is disposed a heating coil 82 for melting theacid crystals through indirect exchange-of heat. 4980 lbs. per hour ofacid crystals at a temperature of 35 F., are passed to melting vessel80. The acid crystals are melted by passing compressed propane vapors atthe rate of 1680 pounds per hour and a temperature of 110 F. fromcompressor 33 through line 84 which is connected to coil 82 andcondensing the propane vapors by passing heat indirectly to the acidcrystals. The liquid propane leaves coil 82 through a line 86 and passesthrough line 24 to eductor 22 previously explained. Additional propanevapors not condensed by the acid melter are condensed by water cooledcondenser 88. Valve 90 controls the flow of liquid returning to theeductor 22. The flow of propane through melter 80 and water cooledcondenser 88 will distribute itself automatically.

The liquid acid present in melting vessel 80 is withdrawn by means of abottom line 92, and it is conveyedv to the alkylation system (not shown)by means of pump 94 and line 18. Any vaporized propane-isobutane whichis present with the acid in the melting vessel is discharged through anoverhead line 96, which in turn is connected to the propane vapor line59. Any propane-isobutane remaining as a liquid is also returned to-thealkylation reactor. .Propane-isobutane needed to replace that lost orreturned to the alkylation plant and to hold the desired ratio ofpropane and isobutane in the system is supplied Via line 97 into line24.

The effectiveness of using a spacer material is illustrated in Table Ibelow. In the examples, filtration was accomplished by creating a vacuumto assist in the passage of uncrystallized liquid through a porousmember. For each example given below, numbers 1-4, inclusive, the cakeof crystals on the porous member was washed with about 30-40 volumes ofwashing agent per volume of acid solids until little or no evidence ofesters or polymers was noticeable in the eluent wash liquid.

l0 fied acid. The data shows that the recovery of acid is increased, andthat the additionally purified acid is of high quality.

Example 7 involved the fortification of spent acid prior to the firstcrystallization. The-results indicate that a substantial yield ofcrystals is obtained by eliminating the free water in the spent acidthrough combination with S03. Furthermore, the crystals are obtainedsubstantially purer than the original spent acid.

The above data clearly demonstrates the advantage of using a spacermaterial as a crystallization medium for the spent acid. The yield ofpurified acid is greatly increased, and when employing this techniquewith centrifugal separation, it is to be expected that a high yield ofcrystals of such high purity will be obtained.

Having thus provided aV description of our invention along with specificexamples thereof, it should be understood that no undue limitations orrestrictions are Table I Example No 1 2 3 4 5 6 6 and 6 7 Acid Samnle Av A A B C, Method .of Separation Filtration Filtration- Centrifugal..Centrifugal. Crystallization Temp., F 0 -30 20 1 30.1 CrystallizingMedium 3 Propanem-- none none none. Washing Agent 2 n dn do Do. SplentAcid Ttratable Acldity, 94.1 94.4 90+SOa=97- 94ISOa=97 ercen AcidCrystals Titratable, Acid- 95.0 98.1 97.9 98.0.

ity, Percent. Mother Liquor Titratable Aeid- 87.8.

ity, Percent. Yields: 3

Percent Liquid 13 30.0 13.4 3.0 5.0. Percent Crystals 87 70.0 86.4 97.095.0.

1 Initial crystallization temperature was 50 F.

2 Where propane is mentioned, it is a mixture of propane and isobutane.

a Yields were determined on an output basis.

It is noted from the data that there is an improvement in the acidcrystal purity by reason that the titratable acidity increased over theValue determined for the original spent acid. Examples 1-5, inclusive,illustrate that the use of low temperatures does effect a purificationof sulfuric acid when using propane as the low viscosity liquid for thecrystallization medium and wash liquid. Further, as the temperature ofcrystallization is decreased, the yield of crystals increasessubstantially; Whereas the purity of crystals decreases slightly as isevident from the titratable acidity. The use of propane as a spacermaterial made possible the employment of low crystallizationtemperatures at which high crystal yields are possible, without anysignificant decrease in crystal purity. Furthermore, by comparison withExample 5, it is to be noted that a much lower yield of crystals isobtained when no spacer is used, and the greater purity of crystals inExample 5 can be attributed to the use of centrifugal separation forrecovery of the acid crystals.

Examples 1-4, inclusive, involved an aged sample of sulfuric acid, thusindicating that our process is feasible for spent acids which have beenstored. As a rule, these acids are more diflicult to purify, because theimpurities are more readily retained by the crystals, probably due toviscosity characteristics.

Example 5 serves to indicate the advantage of-using centrifugal meansfor separating the acid crystals from the mother liquor. In thisexample, neither a crystallization medium nor a wash liquid wereemployed, and yet the results clearly show the improvement in acidpurification by centrifugal separation alone. In practice, centrifugingand supporting uid would be used.

Example 6 is concerned with the fortification of the mother liquor ofExample 5 to recover additional purito be imposed by reason thereof, butthat the scope of the invention is defined by the appended claims.

We claim:

1. In a process for purifying sulfuric acid contaminated with at leastone impurity selected from polar and nonpolar impurities wherein a 'lowviscosity organic liquid which is chemically inert and immiscible withsulfuric acid is passed to a crystallization zone to form a liquid bodytherein, contaminated acid is passed to the crystallization zone, saidcontaminated acid is dispersed into the liquid body in thecrystallization zone which is maintained at a temperature sutiicient tocrystallize sulfuric acid, and a slurry of acid crystals and liquid iswithdrawn from the crystallization zone, the improvements whichco'mprise recycling a lportion of said slurry to the crystallizationzone, separating acid crystals from the remaining portion of saidslurry, and melting said separated crystals to produce an acid ofimproved purity.

2. In a process for purifying sulfuric acid contaminated with at leastone impurity selected from polar and non-polar impurities wherein a lowviscosity organic liquid which is chemically inert and immiscible withsulfuric acid is passed to a crystallization zone to form a liquid bodytherein, contaminated acid is passed to the crystallization Zone, saidcontaminated acid is dispersed into the liquid body in thecrystallization zone which is maintained at a temperature suliicient to`crystallize sulfuric acid, and a slurry of acid crystals and liquid iswithdrawn from the crystallization zone, the improvements which compriserecycling a portion of said slurry to the crystallization zone,separating acid crystals from the remaining portion of said slurry,withdrawing vaporized organic liquid from the crystallization zone andcompressing the same, and contacting the compressed material l 1indirectly with the separated acid crystals thus melting the acidcrystals and condensing the organic liquid.

3. In a process for purifying sulfuric acid contaminated with at leastone impurity selected from polar and nonpolar impurities wherein a lowviscosity organic liquid which is chemically inert and immiscible withsulfuric acid and having a Aboiling point which is lower than theboiling point of the impurity is passed to a crystallization zone toform a liquid body therein, contaminated acid is passed to thecrystallization zone, said contaminated acid is dispersed into theliquid body in the crystallization zone which is maintained at atemperature sufficient to crystallize sulfuric acid, and a slurry ofacid crystals, organic liquid and mother liquor is Withdrawn from thecrystallization zone, the improvements which comprise recycling aportion of said slu'rry' to the crystallization zone, separating acidcrystals from the remaining portion of said slurry, separating organicliquid from the mother liquor, melting the separated acid crystals toproduce an acid of improved purity, and heating `a portion of theseparated organic liquid until substantially all of the organic liquidis vaporized thus leaving a residue containing impurity.

4. -In a process for purifying sulfuric acid contaminated with at leastone impurity selected from polar and nonpolar impurities wherein a lowviscosity organic liquid which is chemically inert and immiscible withsulfuric acid and having a boiling point which is lower than the boilingpoint of the impurity is passed to a crystallization zone to form aliquid body therein, contaminated acid is passed to the crystallizationzone, said contaminated acid is dispersed into the liquid body in thecrystallization zone which is maintained at a temperature suicient tocrystallize sulfuric acid, and a slurry of acid crystals, organic liquidand mother liquor is withdrawn from the crystallization zone, theimprove'ments which comprise recycling a portion of said `slurry to thecrystallization zone near the point of introduction of contaminated acidthereto, separating acid crystals from the remaining portion of saidslurry, separating organic liquid from the mother 12 liquor, melting theseparated acid crystals to produce an acid of improved purity, andheating a portion of the separated organic liquid until substantiallyall of the organic liquid is vaporized thus leaving a residue containingimpurity.

5. In a process for purifying sulfuric acid contaminated with at leastone impurity selected from polar and nonpolar impurities wherein a lowviscosity volatilizable liquid mixture of organic compounds which ischemically inert and immiscible with sulfuric acid is passed to acrystallization zone to form a liquid body therein, contaminated acid ispassed to the crystallization zone, said contaminated acid is dispersedinto the liquid body in the crystallization zone which `is maintained ata temperature sutlicient to crystallize sulfuric acid by vaporizing aportion of the organic compounds, and a slurry of acid crystals, liquidorganic lcompounds and mother liquor is withdrawn from thecrystallization zone, the improvements which comprise recycling aportion of said slurry to the crystallization zone, passing theremaining portion of said slurry to a first separation zone for theseparation of acid crystals, passing liquid from the first separationz'one to a second separation zone to separate liquid organic compoundsfrom mother liquor, washing separated acid crystalls with liquid organiccompounds in a washing zone, passing washed crystals containing organiccompounds to a melting zone to melt the crystals and produce an acid ofimproved impurity, withdrawing vaporized organic compounds from thecrystallization zone, the lirst and second separation zones, the washingzone, and the melting zone, and recondensing the same for further use inthe process.

References Cited in the file of this patent UNITED STATES PATENTS2,254,788 lBallard Sept. 2, 1941 2,593,128 Feuer Apr, 15, 1952 2,716,592Skelly et al Aug. 30, 1955 UNITED STATES PATENT OFFICE CERTIFICATE 0FvCORRECTION Patent Novn 2,863y'722r December 97 1958 Joseph Fu Shelly etal.,

It is herebjST certified that error appears in the-printed specificationof the above v'numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

Column l2, line 28, for Yimjpuritjym read purity een,

Signed and sealed this 21st day of July 195% (SEAL) Attest:

ROBERT C. WATSON Commissioner of Patents KARL H. AXLINE AttestingOfficer

1. IN A PROCESS FOR PURIFYING SULFURIC ACID CONTAMINATED WITH AT LEASTONE IMPURITY SELECTED FROM POLAR AND NONPOLAR IMPURITIES WHEREIN A LOWVISCOSITY OF ORGANIC LIQUID WHICH IS CHEMICALLY INERT AND IMMISCIBLEWITH SULFURIC ACID IS PASSED TO A CRYSTALLIZATION ZONE TO FORM A LIQUIDACID THEREIN, CONTAMINATED ACID IS PASSED TO THE CRYSTALLIZATION ZONE,SAID CONTAMINATED ACID IS DISPERSED INTO THE LIQUID BODY OF THECRYSTALLIZATION ZONE WHICH IS MAINTAINED AT A TEMPERATURE SUFFICIENT TOCRYSTALLIZE SULFURIC ACID, AND A SLURRY OF ACID CRYSTALS AND LIQUID ISWITHDRAWN FROM THE CRYSTALLIZATION ZONE, THE IMPROVEMENTS WHICH COMPRISERECYCLING A PORTION OF SAID SLURRY TO THE CRYSTALLIZATION ZONE,SEPARATING ACID CRYSTALS FROM THE REMAINING PORTION OF SAID SLURRY, ANDMELTING SAID SEPARATED CRYSTALS TO PRODUCE AN ACID OF IMPROVED PURITY.